This invention relates to the gelation method of preparing latex foam rubbers. More particularly, it relates to gelations in which alkali metal fluo-complexes are used as the gelling agents. It is recognized in the industry that these are acid-acting delayed-action gelling agents.
It is known that foam rubber can be prepared from a latex of a rubbery polymer by compounding the latex with the required ingredients, foaming the compounded latex, spreading the foam over a supporting substrate and heating it to set the foam structure, remove the water and form a dry vulcanized layer of foam rubber. See "Latex Foam Rubber" by E. W. Madge, John Wiley & Sons, New York and Maclaren & Sons Ltd., London, 1962, and British Pat. Nos. 598,610 (F. H. Untiedt), 1,342,510 (Firestone Tire and Rubber Co.) and 1,451,130 (Polysar Limited). The gelling agent, when used, is conventionally added just after the foaming step. It functions to destabilize the foamed latex compound and thus cause the rubber particles to join together in the form of a recticulate structure. Sodium silicofluoride (also called SSF) is the most commonly used fluo-complex-type of acid-acting delayed-action gelling agent although fluostannates, fluotitanates and fluozirconates of sodium and potassium have also been proposed. The important characteristic of these delayed-action gelling agents is that as a result of their time-dependent chemical action, they cause progressive destabilization of the foamed latex compound which culminates in its gelation into the reticulate structure. This places a limit on the time within which the foamed compound must be used and places a limit on the amount of gelling agent which can be used. In spread foam applications, e.g. providing fabrics such as carpets with attached layers of foam rubber using sodium silicofluoride as gelling agent, experience has shown that it is difficult to maintain operations at the optimum amount of gelling agent since the "gel range" is too restricted, especially with a highly loaded compound. "Gel range" may be defined as the spread in the amount of gelling agent which can be added to the latex compound and still obtain good foam products. The spread in the gel-range for highly loaded compounds to be gelled by sodium silicofluoride is about 0.1-0.2 part, e.g. at the 2.5 parts SSF level it would be about 2.4-2.6 parts by weight of SSF per 100 parts by weight of latex solids in the compound. This allows very little leeway for variation in the concentration of the sodium silicofluoride solution makeup and in the rate of addition of the sodium silicofluoride solution to the latex compound as it is being fed to the foaming apparatus. Unless the amount of sodium silicofluoride added is controlled very carefully to keep it within the gel-range for the existing foaming conditions, crazing, cracking or areas of collapse are encountered in the resulting foam rubber products. It is desirable to find a method for broadening the gel range for the acid-acting delayed-action gelling agents in general and sodium silicofluoride in particular to avoid these problems.